1. Field of the Invention
The present invention relates to a digital single-lens reflex camera.
2. Description of the Related Art
As a conventional autofocus (AF) technique for digital single-lens reflex cameras, phase-difference type AF (phase-difference AF) is known (for example, see Japanese Patent Laid-Open No. 2004-184952). This phase-difference AF can figure out the defocus amount of a photographing optical system in one-time focus detection operation to enable fast AF with a short release time lag.
In general, in a phase-difference AF system, a light beam that has passed through the photographing optical system is transmitted through a movable mirror and then reflected downward by a sub-mirror arranged behind the movable mirror so that the light beam will be guided to an AF sensor through an AF optical system arranged in the reflection direction of the sub-mirror. Then defocus amount is calculated from the output of the AF sensor. In the conventional single-lens reflex camera, since an image pickup device for capturing a subject image is arranged behind the movable mirror, any light beam for forming the subject image cannot be guided to the image pickup device during the phase-difference AF operation. Thus, since the image pickup device cannot capture a series of subject images continuously during the phase-difference AF operation, the camera cannot provide continuous display of the captured images, so-called live view display, on an LCD monitor or the like.
In other words, in order to display a live view on the LCD monitor or the like, it is necessary to change the position of the movable mirror to an up position retracted from the photographing optical path so that the subject image will be guided directly to the image pickup device. On the contrary, it is necessary to keep the movable mirror at the down position during phase-difference AF, and this makes it difficult to perform phase-difference AF while displaying a live view.
Therefore, a digital still camera is proposed, which can perform contrast type AF (contrast AF) in addition to phase-contrast AF so that either of AF systems can be selected according to the circumstances (see Japanese Patent Laid-Open No. 2001-272593). The contrast AF system is to achieve AF by driving the photographing optical system in such a manner that the contrast of a subject image captured by the image pickup device will become the highest. This contrast AF makes it possible to perform AF while displaying a live view.
However, even using the technique described in Japanese Patent Laid-Open No. 2001-272593, live view display cannot be performed when phase-difference AF is selected. In other words, there is no other choice but to select contrast AF in order to perform live view display and AF simultaneously, and this makes it hard to achieve fast AF.